Support for lead frame for IC chip carrier

ABSTRACT

A support for a lead frame for use with an integrated circuit chip is made of electrically insulative material, preferably a thermoplastic. There is a pad area at the center of the support in which an IC chip can be disposed. Proximate the periphery of the pad are raised spaced-apart projections. The spaces between the projections define slots in which the leads of a lead frame can be disposed.

This invention concerns lead frames for use with integrated circuitchips. The manufacture of such lead frames is disclosed in, for example,the following U.S. Pat. Nos. 4,204,317; 4,137,546; 3,750,277; 3,537,175;3,469,953. After an IC chip has been mounted on a lead frame chipcarrier, and electrically connected to the leads thereof, it is oftenpackaged in encapsulating material. Such packaged chip-containing leadframes are shown in, for example, the following U.S. Pat. Nos.4,331,831; 4,301,464; 4,298,883; 4,264,917; 4,214,364; 4,132,856;4,124,864; 4,079,511; 3,978,516; 3,793,474; 3,684,464; 3,659,821;3,611,061; 3,436,810.

As pointed out in the above patents, the lead frames are usually made bystamping or etching metal strips. Where the manufactured quantities aresufficiently great, stamping is preferred because of its lower cost.

As the circuits on the chips become more complex, a greater number ofleads per lead frame is required. This complicates the stampingoperation. Stamping is carried out in progressive dies; that is, a metalstrip is fed through a stamping press having successive punch and diesets at a number of stations, where successive stamping operations arecarried out. For example, a lead frame having 68 leads would requireabout 15 to 20 stamping stations and about 140 to 150 cuts. As thenumber of required stamping stations and cuts increases, the difficultyin maintaining adequate quality of the stamped lead frames alsoincreases.

It is a purpose of this invention to reduce the number of requiredstamping stations and cuts per lead frame. This is accomplished by notmaking an entire lead frame out of one integral metal strip, as was donein the prior art. Instead, the lead frame is fabricated from a pluralityof segments which can be fabricated from the same set of stamping tools.Thus, if two segments are used per lead frame, the number of stampingstations can be about halved and the number of cuts can be reduced evenmore. And if four segments are used per lead frame, the number ofstamping stations and cuts can be reduced an even greater extent. Thelead frame is manufactured by assembling the segments with precisepositioning, and then securing the segments in a suitable support tomaintain said precise positions. An IC chip can then be disposed thereonand electrically connected to the leads.

This invention is particularly concerned with a support for such a leadframe. Such a support is made of electrically insulative material, has apad area at the center thereof and has raised spaced-apart projectionsproximate the periphery of the pad area. The spaces between theprojections define slots in which the leads of a lead frame can bedisposed.

In the drawing,

FIG. 1 shows a lead frame segment after stamping and FIG. 2 shows astrip of such lead frame segments.

FIG. 3 is a plan view of a support for a lead frame.

FIG. 4 shows the support with three lead frame segments attached and thefourth ready for attachment.

FIG. 5 shows the support with all lead frame segments attached and theleads formed.

FIGS. 6 and 7 are plan and end views of a cover for the support.

FIG. 8 shows the finished chip-containing lead frame package.

In one embodiment, substantially identical lead frame segments 1 werestamped from a long metal strip, as shown in FIG. 2. One such segment 1is shown in FIG. 1. In one example, segments 1 were stamped from 10 milcopper alloy, specifically, Olin CDA194, and were maintained in stripform by means of continuous rails 2 and 3, as shown in FIG. 2. Betweensegments 1, there were support bars 35 extending from rail 3 to rail 2.Each segment 1 comprised seventeen leads 4. The inner ends 5 of leads 4,which would eventually be electrically connected to an IC chip, were 9mils wide and spaced 9 mils apart. The widest parts 6 of leads 4 wereinward from the outer ends 7 thereof and were 28 mils wide spaced 22mils apart. Outer ends 7 were 18 mils wide spaced 32 mils apart. Therewere score lines 8 and 9 on leads 4 about 5 mils from rails 2 and 3.Rails 2 and 3 would eventually be broken off at score lines 8 and 9.There were 62 mil diameter holes 10 in rail 3, which provided means forprecise positioning of the lead frame segments during indexing of thecopper strip during processing. Rails 2 and 3 were 63 and 100 mils wide,respectively. The distance between score lines 8 and 9 was 480 mils.

Support 11 for this example, shown in FIG. 3, was made of a hightemperature thermoplastic material, specifically, a polyetherimide resinmade by General Electric Co., Pittsfield, Mass., under the trade nameUltem. In the center of support 11 was a pad area 12 in which an IC chipwould eventually be disposed. Pad area 12 was 340 mils square and wasrecessed 8 mils below surface 13 of support 11. Surrounding pad area 12were seventy-two raised projectons 14, eighteen per side. Projections 14were 7 mils wide by 25 mils long by 9 mils high. Spaces 15 between theprojections were 11 mils wide. At assembly of segments 1 to support 11,narrow ends 5 of leads 4 would lie on surface 13 in spaces 15 betweenprojections 14. This provides means for accurate co-planar positioningof ends 5 in support 11. On the inner end of each projection 14 wasanother raised portion 16, measuring 7 mils wide by 10 mils long by 6mils high. Raised portions 16 provided means for securing the leads inspaces 15 by, for example, heating and slightly flowing plastic portions16 over the leads. There was an encircling trough 17 outside theperimeter formed by all the projections 14.

At the periphery of support 11 there were sixty-eight slots 18 in whchparts 6 of leads 4 would fit. Slots 18 were formed by projections 29which were similar to projections 14 but bigger. Projections 29 were 20mils wide by 32 mils long by 9 mils high. Slots 18 were 30 mils wide.There were raised portions 30 on the inner ends of projections 29,similar to raised portions 16 on projections 14. Raised portions 30 were20 mils wide by 10 mils long by 6 mils high.

In order to permit supports 11 to be processed in existing manufacturingequipment for chip-carrying lead frames, there were rails 19, compatiblewith said equipment, spaced from each side of support 11 and connectedthereto by tie bars 20. Rails 19 were 10 mils thick by 135 mils wide andwere 1.400 inches apart from each other. Holes 21 provided means foraccurate positioning of the supports during indexing. There were fourposts 22 on support 11 for accurate mating of support 11 with cover 23,which was made of the same plastic as support 11.

Cover 23 had a recess 24 to mate with pad area 12, and had an additionalrecess 25 to accommodate projections 14. Cover 23 also had a trough 26to mate with trough 17, and four holes 27 into which posts 22 fit. Therewere also two holes 28 connecting with trough 26. After assembly ofcover 23 onto support 11, a sealant material could be injected intoholes 28 to fill troughs 17 and 26 in order to seal the IC chip from theenvironment.

Assembly of the components would proceed as follows. Four lead framesegments 1, with rails 2 and 3 still attached, would be placed on top ofsupport 11, one at a time and one in each quadrant, as shown in FIG. 4.Each segment 1, with rails 2 and 3 still attached thereto but with rail2 having been trimmed sufficiently to fit the space, as shown in FIG. 5,would be carefully positioned so that ends 5 lie in spaces 15 and parts6 lie in slots 18. Pressure would then be applied to segment 1 to ensurethat it was flush against surface 13 and then, maintaining the pressure,heat would be applied to portions 16 and 30 to cause them to flow over,and secure in place, ends 5 and parts 6 of leads 4. Rails 2 and 3 wouldthen be broken off at score lines 8 and 9. This process would berepeated for each lead frame segment 1. This construction ensuresaccurate positioning, with secure attachment, of all sixty-eight leadswith excellent co-planarity thereof.

Next, an IC chip 31, shown in dotted lines in FIG. 5, would beconventionally attached in pad area 12, and would be conventionallyelectrically connected to ends 5 of leads 4. The rail construction ofsupport 11 permits this processing to be performed on existingchip-carrier lead frame processing equipment. An advantage of thisinvention is that the IC chip and package can now be tested forelectrical characteristics, because all sixty-eight leads are nowelectrically isolated, and repair of electrical connections can be made,if necessary. In the past, such repair could not be made because theleads were not electrically isolated until after packaging. The externalends of leads 4 are formed into a desired shape, such as "J" lead shape34 shown in FIGS. 5 and 8.

Next, cover 23 is placed on support 11, posts 22 fitting into holes 27.Holes 27 are counterbored about half-way through as shown in FIG. 7, forexample, to 43 mils diameter from 33 mils diameter. Posts 27 extendsomewhat in the counterbores of holes 27, thereby providing the meansfor securing cover 23 to support 11 by heat-flowing the extending upperends of posts 27, as raised portions 16 and 30 were heat-flowed. Next, asealant, for example, a silicone gel, is pumped into holes 28 tocompletely fill troughs 17 and 26 as well as holes 28. The sealantshould be sufficiently adherent and flexible, after curing, to maintaina seal through normal thermal cycling requirements. If desired, assemblyof cover 23 and sealing of troughs 17 and 26 can be performed in anatmosphere having greater heat dissipation characteristics than air,such as helium. This atmosphere will be sealed in the recesses in whichchip 31 is disposed. To complete the package, a lead identifier 32 maybe used, as shown in FIG. 8, and rails 19 and tie bars 20 are removed atscore lines 33.

The finished unit, shown in FIG. 8, had overall measurements, includingformed leads 34, of 988 mils square by 175 mils high.

This invention is also applicable to lead frames made other than bystamping, such as etching or chemical forming. There, a significantadvantage to the use of segments as per this invention is a substantialreduction in tooling cost.

We claim:
 1. A lead frame support comprising a support made of anelectrically insulative, thermoplastic material, a pad area at thecenter of said support in which an IC chip can be disposed, said padarea being square or rectangular, and first raised spaced-apartprojections proximate the periphery of said pad area, on all four sidesthereof, the spaces between the projections defining slots in which theleads of a lead frame can be disposed.
 2. The lead frame support ofclaim 1 wherein said leads of said lead frame can be secured in saidslots by heating and slightly flowing said projections over said leads.3. The lead frame support of claim 2 wherein there is a trough in thesupport encircling said pad area and said first projections.
 4. The leadframe support of claim 3 comprising, in addition, second raisedspaced-apart projections proximate the periphery of said support, thespaces between said second projections defining slots in which saidleads of said lead frame can be disposed.
 5. The lead frame support ofclaim 4 wherein said pad area is recessed.
 6. The lead frame support ofclaim 5 wherein said support has four sides and there are removablerails at only two of said sides, said rails being oppositely disposedfrom one another and being spaced from, and connected to, said supportby tie bars.